An optical information recording medium having a recording layer is practically used as a rewritable optical disk such as CD-RW or DVD-RW. Such an optical information recording medium usually takes a four layer structure comprising a protective layer, a recording layer, a protective layer and a reflective layer on a substrate. Here, it is common to employ an Al alloy for the reflective layer.
With those presently practically used as rewritable optical information recording media, a crystallized state corresponds to a non-recorded or erased state, and an amorphous non-crystallized mark corresponds to a recorded state. The recorded i.e. amorphous marks can be formed by heating the recording layer to a temperature higher than the melting point, followed by quenching. On the other hand, the erased i.e. crystallized state can be formed by heating the recording layer to a temperature which is higher than the crystallization temperature of the recording layer and lower than the melting point.
As mentioned above, the recording layer is heated and cooled at the time of recording and erasing, and due to such thermal history, the recording layer is likely to undergo evaporation or deformation. Accordingly, in order to prevent evaporation or deformation of the recording layer, it is common to sandwich the recording layer between highly heat resistant and chemically stable protective layers. Further, such protective layers promote thermal diffusion from the recording layer during the recording and bring the recording layer to a super cooled state, and thus it contributes to formation of an amorphous mark.
Further, a metal reflective layer is formed on the structure having the recording layer sandwiched between the protective layers, whereby due to an optical interference effect, the signal modulation degree at the time of playback will be increased, the thermal diffusion will be further promoted to further stabilize formation of amorphous marks, and further, the erasing characteristics or repeated recording characteristics will be improved.
Here, if a so-called rapid-cooling structure is taken, for example, by reducing the thickness of the protective layer between the recording layer and the reflective layer, heat dissipation of the recording layer will be further promoted, whereby formation of amorphous marks will be further facilitated, and the high speed recording, the recording sensitivity and the durability against repeated recording can be improved. Further, in order to realize the high speed recording, it is studied to change the material for the reflective layer from a conventional Al alloy to a material containing Ag as the main component which has a higher heat dissipation effect. By using Ag, heat dissipation of the recording layer will be further promoted, and it becomes possible to form amorphous marks at a high speed.